This invention relates to electrical testing of electronic circuits and in particular improved methods and apparatus for testing electronic circuits such as those existing in chip carriers, printed circuit boards, substrates and other microcircuit devices.
Flaws resulting in manufacturing processes of microcircuit devices can create undesired electrical conditions in electrical circuits contained within such devices resulting in flawed or inoperable electrical circuits wherein the flawed circuits do not function as originally intended and designed. In order to verify the electrical integrity of electronic circuits such as those present in chip carriers, printed circuit boards, electronic substrates or other microcircuit devices, during manufacture of such devices such circuits are typically submitted to electrical testing in an effort to detect unintended design flaws such as conditions of electrical shorts or opens in the circuits.
There is a significant ongoing design and manufacturing effort within the electronics industry to create physically smaller and more compact electronic components and substrates. When attempting to make smaller substrates, the density of the electronic circuits on the substrates increases and it is a requirement that the pads on the substrates be placed much closer together compared to substrates using less dense electronic circuits. With increases in electronic circuitry density the contact pads, which represent extremities of the circuits are spaced ever-closer together, especially in the region of the substrate where relatively smaller chips or components are to be subsequently connected. This causes difficulties in effecting contact of the desired individual circuit contact pads on the surface of the substrate for purposes of detecting undesired electrical conditions in the individual circuits.
Typical substrates, for example, chip carriers, printed circuit boards, electronic substrates, or similar devices, comprise electronic circuits and the substrates physically and spatially support the electronic circuits. A substrate comprises non-conductive material so that it does not interfere with the normal operation of the electrical circuits supported thereon. These arrangements are well known in the art. The substrate typically includes electronic circuits and a first group of pads located on one surface of the substrate, with the pads electrically connected to one or more of the electronic circuits. The pads are used to connect the electronic circuits to a first electronic device, for example, an electronic chip. A second group of pads located on another surface of the substrate, are electrically connected to the electronic circuits and intended to connect the electronic circuits to a second electronic device, for example, a printed circuit board. The electronic circuits form a plurality of predetermined conductive pathways between the first and second groups of pads on the substrate.
A known approach used for testing and detecting undesired electrical conditions such as open circuits, where contact pads are relatively closely spaced is referred to as electrical-commoning. With this approach, all the contact pads of one group of pads which represent all the first extremities of the electronic circuits of the substrate are electrically shorted. The electronic circuits are all thus effectively shorted and connected together. Test probe devices are contacted to the second group of pads which represent the other extremities of the electronic circuits and which pads are typically spaced further apart from each other than those pads at the first extremities of the circuits. The test probes are connected to a test mechanism for testing the electronic circuits for adverse electrical conditions and verify whether the electronic circuits exhibit any adverse electrical conditions. Electrical continuity-testing of electrical circuits may be carried out with the use of, for example: an ohm meter, a voltmeter, an ammeter, or equivalent, in order to verify the electrical integrity of electronic circuits by detecting undesired electrical conditions in the circuitry, as is well known in the industry.
Problems exist resulting from the use of known electrical-commoning methods and apparatus for detecting undesired electrical condition of electronic circuits on substrates which could produce “false open” results. These problems include:
the electrical-commoning mechanism does not making suitable contact with the contact pads;
the pads are contaminated, for example, by a non-conductive substance on the pads which substantially prevents electrical contact being made between the pads and the electrical-commoning mechanism;
one or more pads may be deformed, thus creating a condition where the pad does not make contact with the electrical-commoning mechanism;
the electrical-commoning means is not sufficiently locally compliant with the pads resulting in ineffective electrical contact;
the electrical-commoning mechanism does not have a capability to overcome aspects of “self-damage” that may be reflected in the contacts so that a reliable commoning of the contact pads can be effected, for example, the self-damage being caused by contaminants, or the pads having surface inconsistencies, or the electrical-commoning mechanism having surface inconsistencies; and
the electrical-commoning means may deposit an unwanted residual electrically non-conductive material on the surface of the pads and the substrate, such that it may be difficult to remove the residual material from the pads and the substrate after testing is performed.
Known prior art attempts to solve the above and other problems associated with electrical-commoning approaches presently used include the use of a flexible sheet, for example, a conductive polymeric sheet or metallized polymeric sheet as described in U.S. Pat. No. 5,898,311 issued Apr. 27, 1999, entitled Shorting Pad Having A Flexible Conductive Sheet and U.S. Pat. No. 5,900,316 issued May 4, 1999, entitled Flexible Conductive Sheet, both of which are assigned to International Business Machines Corporation. However, difficulties may arise with the use of such polymeric sheets in that:
the sheet may leave a residual contamination on the surface of the pads and substrate during the test which may be difficult to remove;
the sheet has a capability of being susceptible to physical damage when contacting the pads;
the sheet requires relatively large contact forces to ensure that the polymeric sheet makes sufficient contact with all of the pads and the large forces may result in damage to the substrate and any test fixture; and
the sheet may be insufficient to completely prevent “false opens” because the polymeric sheet does not have sufficient capability for being substantially and completely locally compliant with the contact pads.
From the above description, it should be appreciated that there is a need for improvements in the testing of electrical circuits that are manufactured on substrates and in particular a need for a new approach to temporarily electrically common or short such circuits. A novel and unobvious solution is needed to provide an electrical-commoning mechanism that overcomes the aforementioned problems inherent in the prior art.